Do the groundwork - create a shape

First of all you need a class derived from the KoShape class. This will be the actual shape class so you have to ensure that all the data you need for painting is accessable for this KoShape derived class.
The only method you have to reimplement is the paint() method which is responsible for painting your shape.

What you might be interested in is the resize() and size() method which set the available size to the shape. Some shapes define the size they need on their own so they reimplement size() to return the size they have.

An example for this is the KoFormulaShape. A formula has a fixed size due to its contents and so KoFormulaShape reimplements the size() method.
If your shape has a special outline, reimplement outline() to return your shape's outline correctly.

The rotation, scaling and skewing is done through a matrix and so you don't need to care about it.

Make your shape loadable - create a factory and a plugin

Now when you have created your shape class and implemented all the necessary things to make it at least compile you can think about the loading of your shape.

The KOffice apps use KoShapeFactory to get instances of shapes in a generic way. This way of obtaining shape instances is designed after the factory pattern. So you should also implement a KoShapeFactory derived class that makes creating new instances of your shape possible. The factory class has to implement two methods:

With the factory there is now a generic way to obtain an instance of your shape. But somehow you have to publish your shape as a plugin to let the KOffice application know that there is a plugin to load. Therefore the flake library provides the KoShapeRegistry class. Each application has access to the registry and to let the application know about your shape you have to register it within the registry.

To register we would have to make a call like this:
KoShapeRegistry::instance()->add( new FooShapeFactory( parent ) );
In order to make that call, we will create a plugin class which is special in that it will be automatically loaded and created by KOffice when an application starts. This means that the constructor of our plugin class will be the perfect place to actually register our shape using the above line.

The registration is done within the constructor of the FooShapePlugin class. This is a very simple class that represents the plugin and does registration.

Example plugin class definition:

include <QObject>

class FooShapePlugin : public QObject {

Q_OBJECT

public:

FooShapePlugin(QObject *parent, const QStringList&);

};

Example plugin class implementation:

include "FooShapePlugin.h"

include <kgenericfactory.h>

K_EXPORT_COMPONENT_FACTORY(fooshapelibrary,

KGenericFactory<FooShapePlugin>( "FooPlugin" ) )

FooShapePlugin::FooShapePlugin(QObject *parent, const QStringList&)

: QObject(parent)

{

// register the shape's factory
KoShapeRegistry::instance()->add(
new KoFooShapeFactory( parent ) );
// we could register more things here in this same plugin.

}

include "FooShapePlugin.moc"

This demonstrates how for the plugin related tasks KDE provides services which are dynamic loaded libraries. The secret ingredient that makes this class the plugin of that library is the call to the K_EXPORT_COMPONENT_FACTORY macro defined in kgenericfactory.h

We now have a way to create instances of your shape (FooShapeFactory), a way to register them for the apps (KoShapeRegistry) and a plugin that can dynamically be loaded (FooShapePlugin). The last step is to create a .desktop file that describes your plugin and makes it findable by KOffice. For "X-KDE-Library" you have to set the library name you have already specified within K_EXPORT_COMPONENT_FACTORY().

After installing that file in the KDE services directory and installing your plugin where the application can open it, your plugin is system wide known and can be loaded by KOffice.

TODO; add example CMake file.

Make your shape editable - create a tool

To edit your shape in the GUI the user wants to select a tool and alter your shape. Therefore you have to provide a KoTool derived class. This class implements all edit actions that can be done on your shape but it is also possible to have more than one tool per shape.